Re: [Flightgear-devel] Helicopter: First Impressions

[Maik Justus]

Josh Babcock schrieb: Martin Spott wrote: Josh Babcock wrote: I like your todo list. I would just suggest one more thing. If you look at a helo from behind, the rotor disk will be canted slightly to one side in straight and level flight. The side force offsets the side force of the tail rotor to prevent sideslip. In FG, instead of the rotor disk being tilted to the left a few degrees, the whole aircraft is. This might very well be a phenomenon that's unique to helicopters which are based on this hingeless rotor concept, aircraft and rotor mostly move as one entity. They vary significantly from the typical Bell rotor style: http://www.christoph31.de/cgi-bin/MasterFrameReunion.cgi?http%3A//www.christoph31.de/foto/details.php%3Fimage_id%3D590%26 Martin. Yeah, this behavior may be appropriate for the bo105, I have a picture of an EC-135 with a semi-rigid rotor in a 1" hover that has that has that same tilt, though it might have just been wiggling as it lifted off. Most of the rotors out there, however, are still fully articulated. There are even a significant number of teetering rotors still flying. Just about every Jet Ranger has one. It is definitely not right for them. As far as I can tell, YASim treats all rotors this way. Josh I think the simulation is correct at this point. But there is a second source for the tilting of the fuselage. The tail-rotor itself gives some moment to the fuselage, depending on the height where the tail-rotor is connected. (This effect is simulated as well.) I've never calculated the strength of this effect, but I expect it to be not negligible for the Jet Ranger. A UH1 I expect to hover more ore less parallel to the ground (depending on the cg). If we trust the simulation (I do), we can check this by using the bell206-simulation and modify the height of the tail-rotor to be the same as the main-rotor. In a RL teetering system for instance, the rotor does not transmit any pitch/roll torque to the mast at all until the hinge reaches its limits, it just tugs it fore and back and side to side. This is only correct for rotor heads with the hinge exactly in the center of the head (like Jet Ranger, UH1). If the hinge is not in the center (e. g. CH53), the rotor transmit torque to the mast. The rotor head of the Bo can transmit more torque to the mast, than the mast  is designed for. Therefor the Bo has an instrument, which shows the moment on the mast. I have some documentation on the bo105 here. There is a comparison of the bo105 and the bell206. If you put the stick to one side the bell206 needs 1.7s for reaching 63% of the final rotational speed. The sa341 (gazelle) needs 1.2s and the bo105 needs 0.2s. Therefore the bell206 reacts very delayed to any input on the cyclic and the bo reacts instantly.   Maik

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